Various position encoders for sensing linear, rotary or angular movement are currently available. These encoders are generally based on either inductive transducers, capacitive transducers, optical systems, or magnetic scales. In general, an encoder may comprise a transducer with a readhead and a scale. The readhead may comprise a transducer element and some transducer electronics. The transducer outputs signals vary as a function of the position of the readhead relative to the scale along a measuring axis. The transducer electronics outputs the signals to a signal processor or processes the signals internally before outputting modified signals indicative of the position of the readhead relative to the scale. It is also common for an encoder system to include an interface electronics separate from the readhead, and to interpolate or otherwise processes the transducer signals in the interface electronics before outputting modified signals indicative of the position of the readhead relative to the scale to an external host system such as a motion control system or data acquisition system.
Optical position encoders, both rotary and linear, use optical detectors to sense position. The optical elements in some of these types of encoders have been designed to produce electronic output signals in phase quadrature. In certain types of these systems, an optical readhead outputs two continuous analog sine wave signals that vary in the amplitude relationship between their phase quadrature signals as the position changes. The scale pitch λ0 of an optical scale is the fundamental wavelength of the optical position encoder system. In one common type of example optical encoder, the scale pitch λ0 may be in the range of 20 to 40 micrometers. In such systems the analog quadrature signals are frequently connected to an electronic signal interpolation device. The output of this device is typically two digital quadrature signals derived from the two continuous analog sine wave signals. These digital quadrature signals are then in a form suitable for use by subsequent digital processing electronics that determine and/or accumulate position changes of the optical encoder. In another type of traditional optical detector, the interpolating electronics may be incorporated inside the readhead.
In such optical encoder systems the analog signals are produced from the optical transducer signals, and the digital signals are created by the interpolation electronics. The analog sine waves have the scale pitch λ0 An example of the most simple type of interpolation is commonly referred to as 4X interpolation. In this case, the analog quadrature signals have been changed into digital quadrature waveforms with the same pitch or wavelength λ0. The term 4X is used because for each wavelength traversed, there are 4 edges, or transitions, of the digital signals. The subsequent electronics are capable of detecting these 4 edges, and thus can detect or record the optical encoder position with a resolution of R1=λ0/4. As an example of a more advanced type of interpolation, the relationship between the analog signals may be interpolated by an extra factor of 4. In this case, the subsequent electronics can record the position with a resolution of R2=λ0/16. In general, in certain example systems interpolation electronics of this type have interpolation factors ranging from 4 to 400, or more.
The nature of the optical systems described above are such that the analog waveforms from the position transducer are continuous with position and time. There are no interruptions in such analog. Because of the continuous nature of these signals, the subsequent interpolating electronics generally continuously derives and outputs the corresponding interpolated digital output signals by continuously processing the continuous analog waveforms.
In contrast, U.S. Pat. Nos. 6,005,387, 6,049,204 6,400,138 and 6,329,813, each incorporated by reference in their entireties, disclose incremental and absolute inductive position transducers, or encoders, that operate on a sampled basis. In such encoders, the absolute position is sampled and/or determined on an intermittent or periodic basis, where the sample period in some systems is typically in the range of 100–200 microseconds or more. At times between the discrete position samples, no new position information is available. Such “interrupted” or “sampled” signals are not conventional in many motion control systems and are not familiar to many users of position encoders. Accordingly, it is not easy for some users to implement such encoders in various applications. For example, in order to be compatible with existing control systems, in various applications the output from such “position sampling” inductive devices would preferably be in the form of continuous digital quadrature signals similar to those of the traditional optical position encoders. However, it is more difficult to create accurate two phase, continuous digital quadrature signals from the sampled signals of such inductive encoders and the like than from continuous analog signals, such as those from an optical encoder, or the like.
The present invention is directed to providing a system and method that overcome the foregoing and other disadvantages limiting the use of position sampled inductive encoders and the like. More specifically, the present invention is directed to a system and method for generating position outputs at a higher rate than the native underlying sample rate of such position sampled encoders, for example, in the form of continuous incremental quadrature signals.